The invention relates to the field of monitoring the loads which occur in roofs, in particular in flat or more or less flat roofs. As is known, such roofs may be subjected to highly variable loads, in particular during rainfall and snowfall. In case of rain, the load on a roof may rise to extremely high unpredictable values, for example due to the drains becoming blocked. However, high loads may also occur with snow, as the snow blanket can become quite thick with continuous snowfall. Although such loads are taken into consideration during the design of the roof, experience has repeatedly shown that flat roofs in particular are ultimately not able to withstand the extreme conditions which may occur in the abovementioned cases.
As large numbers of people may be present in the respective buildings, such loads are a great risk. Examples of such buildings which may be mentioned are swimming pools, furniture stores, schools and the like. The collapse of the roof of such public buildings may therefore have serious consequences. For this reason, risk assessments are being carried out on an increasing number of buildings, during which the susceptibility of the roof to overload is established. As a rule, such assessments result in the respective buildings having to be modified, such as by the installation of emergency overflow systems for a reliable discharge of precipitation and the like.
However, until now none of these measures have resulted in a reliable reduction in the risk. By way of example, the provision of emergency overflow systems is mentioned, which, in practice, have been found to provide only a partial solution. Although such systems are able to drain large amounts of water in a reliable manner, they do not offer a solution for small leaks which result in the accumulation of increasing amounts of water in the roof insulation. As soon as the roof insulation is saturated, the total weight of the accumulated water, and thus the load on the roof, may become so great that the risk of collapse still emerges.
Snowfall causes even greater problems, as snow cannot be easily removed. In addition, there is the added risk of snow which has partially melted freezing, thus blocking the drains with ice. During a subsequent thaw with, for example, simultaneous rainfall, the risk of collapse increases in this case as well.
U.S. Pat. No. 5,850,185 discloses a method for monitoring the load which is exerted on a roof, comprising the following steps:                providing a roof with an external surface on which a load can be exerted by, for example, a layer of water or snow,        providing a sensor for detecting a phenomenon related to the load which is exerted on the roof.        
Although this known method is able to provide an indication with respect to the relevant potential dangers, the risk that any warning may be too late still exists. The fact is that if the load which is exerted on the roof increases rapidly, there may be insufficient time to completely evacuate the relevant area or building. It is possible to achieve a longer period for evacuation by already providing the warning at a very early stage, which, however, has the drawback that many premature alarms will be raised, most of which will turn out to be false. With this known method, it is thus not possible to create a balance between, on the one hand, a reliable warning, and, on the other hand, a sufficiently long evacuation period.
Therefore, it is an object of the invention to provide a solution for these problems in connection with providing warnings concerning unexpectedly high loads on flat or virtually flat roofs. In particular, it is an object of the invention to achieve a significant reduction in the risk which such unforeseen loads form for the public inside the building concerned. This object is primarily achieved by means of a method for monitoring the load which is exerted on a roof, comprising the following steps:                storing at least two predetermined different values for the phenomenon,        emitting a signal when the sensor detects one of these values for the phenomenon,        emitting a different signal when the sensor detects another one of these values for the phenomenon.        